1. Field of the Invention.
The present invention relates to heater plates, and more particularly, to heater plates for heat printing packaging materials such as, for example, carriers and covers for containers.
2. Description of Related Art.
A method and apparatus for "heat printing" carriers or covers for a plurality of containers is described in U.S. Pat. No. 4,688,367 by Bonkowski. As described therein, a carrier, for packaging an array of articles as a single unit, such as the common six-pack beverage can container, may be formed by placing a sheet of thermoplastic material near or upon a heated platen or anvil. The platen typically has a generally flat contact area which transfers heat to the sheet ("heat printing") to soften the area in contact or near proximity to the heated platen. After a portion of the sheet has been softened in appropriate areas, the sheet is transferred to a position adjacent to the containers for stretch forming the sheet over the containers. A forming device draws the softened portion of the sheet, along with any nonsoftened areas bordering the softened portion, over and around a surface of the container or package intended to be covered or held, thereby stretching the softened areas. The softened portion of the sheet is then allowed to cool to its unsoftened, relatively rigid state, resulting in a carrier that covers the top and portions of the sides of each article.
The above described heat printing method and apparatus has been found to provide a very effective cover and carrier for packaging articles such as containers. However, one problem that has been encountered is the tendency for small pockets of air to become trapped between the contact surface of the platen and the thermoplastic sheet to be softened. These air pockets can act as an insulating film to inhibit the transfer of heat from the platen to the sheet. As a result, small portions of the sheet may not become sufficiently softened to properly stretch over the top of the container. Consequently, the carrier may not properly seal the top of the container. Also, undesired holes may develop in the top of the sheet.